1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for performing printing in two designated colors, for example, in a digital copying machine having a CCD that reads red, green and blue signals and a monochromatic signal.
2. Description of the Related Art
In the field of digital copying machines, an image processing apparatus has been proposed, which effects printing of a read original image using two colors (e.g. red and black) designated by a user through designation means such as a control panel. In this case, the main purpose of the two-color printing is to extract a red region on an original, print the extracted red region in red, and print the other color region in black. The two-color printing thus requires means for separating a red region and a black region.
As a technique for the image processing apparatus for two-color printing, a method has been proposed, wherein input R, G, and B signals are directly converted to a red signal and a black signal, which are output color signals, by a matrix arithmetic operation. In the case where signal values (colors) of C (cyan), M (magenta), Y (yellow) and K (black) corresponding to the red and black are directly calculated from the R (red), G (green) and B (blue) signals by the matrix arithmetic operation, C and K colors, which are unnecessary colors, mix in the calculated red signal, resulting in an impure red color. Similarly, unnecessary C, M, and Y colors mix in the calculated black signal, resulting in an impure black color.
Such impure colors occur due to the difficulty in reproducing single colors of red and black by CMYK signals, relative to 256-gradient data of each of RGB signals, and in finding matrix coefficients that do not cause tone inversion.
Jpn. Pat. Appln. KOKAI Publication No. 8-84268 discloses a method for solving the problem. This method includes a process of subjecting RGB data input by a scanner to a density conversion to produce a monochromatic signal (density signal). The method also includes a process of color-converting RGB data to CMY data, following which a hue determination process and a saturation determination process are performed. Based on the determination results of hue and saturation, it is determined whether red-color printing or black-color printing is to be performed in units of a pixel. The printing with the determined print color is effected using the density data obtained by the density conversion.
This method, however, requires a circuit for determining hue and saturation, leading to an increase in cost. Moreover, since the signals are separated into red and black and are directly used for printing, it is not possible to switch a process for a character region in an image and a process for a photo region in the image.